This invention relates to the field of tactile or snap action keyboards. More particularly, this invention relates to the field of tactile or snap action elements which are in the form of protrusions in a plastic sheet. As is well known in the art of tactile or snap action membrane keyboards, tactile or snap action protrusions may be formed in a sheet of Mylar (a trademark of E. I. DuPont DeNamours and Co.) material or other suitable plastic material. The protrusions are sometimes referred to as "bubbles", although several different geometric configurations of the "bubbles" are known in the art. By way of example, U.S. Pat. No. 3,643,041 to Jackson discloses a snap action keyboard in which the protrusions are in the form of semi-spherical domes; U.S. Pat. No. 3,860,771 to Lynn et al shows a snap action keyboard in which the key elements are in the form of semi-spherical domes located on top of cylindrical pedestals; and U.S. Pat. No. 4,190,748 to Langford discloses a tactile keyboard in which the tactile key elements are in the form of truncated cones. While those three patents are by no means a complete list of all prior art in the field of tactile keyboard elements, they do illustrate what are believed to be the most commonly used geometric configurations for these tactile key elements.
Although the geometric configurations of the tactile key elements are different in the three patents cited above and other geometric configurations may exist, a characteristic believed to be common to all geometric configurations of tactile key elements is that they are symmetrical. Thus, in the three patents identified above, the tactile key elements are, when viewed in cross section, symmetric with respect to a center axis through the elements; and in these three cases at least, the tactile elements are also symmetric surfaces of revolution about their center axes. As stated, this characteristic of symmetry (either in cross section or in surface of revolution) is believed to be incorporated in all tactile or snap action key elements of membrane type keyboards.
A problem often encountered with prior art tactile or snap action membrane keyboards is that the consistency of snap action or tactile feel (or tactile feedback as it is sometimes called) may be very sensitive to the place and manner of application of the actuating force. Many of these snap action protrusions or bubbles require actuation essentially at the center of the bubble to obtain proper and consistent snap action, while others, such as the configuration shown in Langford U.S. Pat. No. 4,190,748, may be actuated at an off center design location; but in all cases, the quality and consistency of snap action is very sensitive to the location at which the actuating force is applied. If the actuating force is not applied within the design tolerances of the actuating point, or if the location of the actuating force is applied inconsistently, inconsistent and often unacceptable (sometimes bordering on nonexistent) snap action or tactile feel may result. As a result of this sensitivity of prior art snap action key elements to the location of the actuating forces, prior art keyboards of this type have been very sensitive to manufacturing and assembly tolerances; and unacceptable products may result if manufacturing tolerances or alignment tolerances in assembly are exceeded. The problem of inconsistency of snap action or tactile feedback is particularly present in keyboards which have a relative thick or stiff overlay sheet, such as an electroluminescent panel or an embossed overlay sheet. The sensitivity of these prior art tactile keyboards therefore complicates both the assembly and operation of these keyboards and increases their expense.